1. Field of the Invention
The present invention relates to (1) a novel ester compound, (2) a polymer comprising units of the ester compound which is blended as a base resin to formulate a resist composition having a high sensitivity, resolution and etching resistance, and in particular, suitable as micropatterning material for VLSI fabrication, (3) a method for preparing the polymer, (4) a resist composition comprising the polymer, and (5) a patterning process using the resist composition.
2. Prior Art
While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 xcexcm or less.
For resist materials for use with a KrF excimer lasers, polyhydroxystyrene having a practical level of transmittance and etching resistance is, in fact, a standard base resin. For resist materials for use with ArF excimer lasers, polyacrylic or polymethacrylic acid derivatives and polymers comprising aliphatic cyclic compounds in the backbone are under investigation. In either case, the basic concept is that some or all of alkali soluble sites of alkali soluble resin are protected with acid labile or eliminatable groups. The overall performance of resist material is adjusted by a choice from among a variety of acid eliminatable protective groups.
Exemplary acid eliminatable protective groups include tert-butoxycarbonyl (JP-B 2-27660), tert-butyl (JP-A 62-115440, JP-A 5-80515, and J. Photopolym. Sci. Technol. 7 [3], 507 (1994)), 2-tetrahydropyranyl (JP-A 2-19847, 5-80515 and 5-88367), and 1-ethoxyethyl (JP-A 2-19847 and 4-215661). While it is desired to achieve a finer pattern rule, none of these acid eliminatable protective groups are deemed to exert satisfactory performance.
More particularly, tert-butoxycarbonyl and tert-butyl are extremely less reactive with acids so that a substantial quantity of energy radiation must be irradiated to generate a sufficient amount of acid in order to establish a difference in rate of dissolution before and after exposure. If a photoacid generator of the strong acid type is used, the exposure can be reduced to a relatively low level because reaction can proceed with a small amount of acid generated. However, in this event, the deactivation of the generated acid by air-borne basic substances has a relatively large influence, giving rise to such problems as a T-top pattern. On the other hand, 2-tetrahydropyranyl and 1-ethoxyethyl are so reactive with acids that with the acid generated by exposure, elimination reaction may randomly proceed without a need for heat treatment, with the result that substantial dimensional changes occur between exposure and heat treatment/development. Where these groups are used as protective groups for carboxylic acid, they have a low dissolution inhibiting effect to alkali, resulting in a high rate of dissolution in unexposed areas and film thinning during development. If highly substituted polymers are used to avoid such inconvenience, there results an extreme drop of heat resistance. These resins fail to provide a difference in rate of dissolution before and after exposure, resulting in resist materials having a very low resolution.
Therefore, an object of the present invention is to provide (1) a novel ester compound capable of forming an acid-decomposable polymer, (2) a polymer which is blended as a base resin to formulate a resist composition having a higher sensitivity, resolution and etching resistance than conventional resist compositions, (3) a method for preparing the polymer, (4) a resist composition comprising the polymer as a base resin, and (5) a patterning process using the resist composition.
The inventor has found that a novel ester compound of the following general formula (1) obtained by the method to be described later is useful in preparing an acid-decomposable polymer; that a resist composition comprising as the base resin a novel polymer prepared from the ester compound to a weight average molecular weight of 1,000 to 500,000 has a high sensitivity, resolution and etching resistance; and that this resist composition lends itself to precise micropatterning.
In a first aspect, the invention provides an ester compound of the following general formula (1). 
Herein R1 is hydrogen, methyl or CH2CO2R3, R2 is hydrogen, methyl or CO2R3, R3 is a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, k is 0 or 1, Z is a divalent hydrocarbon group of 2 to 20 carbon atoms which forms a single ring or bridged ring with the carbon atom and which may contain a hetero atom, m is 0 or 1, n is 0, 1, 2 or 3, and 2m+n is 2 or 3.
In a second aspect, the invention provides a polymer comprising recurring units of the following general formula (1a-1) and/or (1a-2) derived from an ester compound of the general formula (1), and having a weight average molecular weight of 1,000 to 500,000.
Herein R1, R2, Z, k, m and n are as defined above.
In a preferred embodiment, the polymer further comprises recurring units of at least one of the following formulas (2a) to (10a). 
Herein R1, R2 and k are as defined above; R4 is hydrogen or a carboxyl or hydroxyl-containing monovalent hydrocarbon group of 1 to 15 carbon atoms; at least one of R5 to R8 is a carboxyl or hydroxyl-containing monovalent hydrocarbon group of 1 to 15 carbon atoms, and the remainders are independently hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, or R5 to R8, taken together, may form a ring, and when they form a ring, at least one of R5 to R8 is a carboxyl or hydroxyl-containing divalent hydrocarbon group of 1 to 15 carbon atoms, and the remainders are independently a single bond or a straight, branched or cyclic alkylene group of 1 to 15 carbon atoms; R9 is a monovalent hydrocarbon group of 3 to 15 carbon atoms containing a xe2x80x94CO2xe2x80x94 partial structure; at least one of R10 to R13 is a monovalent hydrocarbon group of 2 to 15 carbon atoms containing a xe2x80x94CO2xe2x80x94 partial structure, and the remainders are independently hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, or R10 to R13, taken together, may form a ring, and when they form a ring, at least one of R10 to R13 is a divalent hydrocarbon group of 1 to 15 carbon atoms containing a xe2x80x94CO2xe2x80x94 partial structure, and the remainders are independently a single bond or a straight, branched or cyclic alkylene group of 1 to 15 carbon atoms; R14 is a polycyclic hydrocarbon group of 7 to carbon atoms or an alkyl group containing such a polycyclic hydrocarbon group; R15 is an acid labile group; X is xe2x80x94CH2xe2x80x94 or xe2x80x94Oxe2x80x94; and Y is xe2x80x94Oxe2x80x94 or xe2x80x94(NR16)xe2x80x94 wherein R16 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms.
In a third aspect, the invention provides a method for preparing a polymer comprising the step of effecting radical polymerization, anionic polymerization or coordination polymerization between an ester compound of the general formula (1) and another compound having a carbon-to-carbon double bond.
In a fourth aspect, the invention provides a resist composition comprising the polymer defined above, and more specifically, a resist composition comprising the polymer defined above, a photoacid generator, and an organic solvent.
In a fifth aspect, the invention provides a process for forming a resist pattern comprising the steps of applying the resist composition defined above onto a substrate to form a coating; heat treating the coating and then exposing it to high-energy radiation or electron beams through a photo mask; and optionally heat treating the exposed coating and developing it with a developer.
The ester compound of formula (1) and the polymer comprising units of formula (1a-1) and/or (1a-2) employ a cycloalkylcycloalkyl or cycloalkylcycloalkenyl group of the following formula (1b) as the acid-eliminatable protective group, thereby overcoming the drawbacks of the tert-butoxycarbonyl and tert-butyl groups having low reactivity with acid as well as the 2-tetrahydropyranyl and 1-ethoxyethyl groups having too high reactivity with acid and low resistance to alkaline developers. 
Herein, Z, m and n are as defined above, the broken line denotes a valence bond.
The ester compound of formula (1) is decomposed under acidic conditions while generating an olefin compound and a carboxylic acid. This mechanism itself is apparently identical with the decomposition of tertiary alkyl esters such as tert-butyl esters although the decomposition rate significantly differs between them. In the acidolysis of a tertiary alkyl ester, carboxylic acid and a tertiary alkyl cation generate first, then the extinction of the cation due to proton release occurs to form an olefin compound, whereby the reaction comes to an end. The rate-determining stage of this decomposition reaction is the late stage of cation extinction. The progress of this stage is very rapid in the case of the ester compound of formula (1), probably because the moderated distortion of the ring and the formation of a conjugated system produce a strong thrust. The olefin compound produced has a very low re-reactivity. As a consequence, the ester compound of formula (1) provides a high degree of acidolysis surpassing other tertiary alkyl esters.
Since the ester compound of formula (1) is basically a tertiary alkyl ester, it, when formulated into a resist composition, does not allow for the infinite progress of decomposition in a duration between exposure and development and film thinning during development as found with 2-tetrahydropyranyl and 1-ethoxyethyl. When a polymer comprising constituent units derived from the compound is formulated as the base resin, there is obtained a resist composition which possesses a high sensitivity and resolution and has a high dissolution contrast in that the unexposed areas have a very slow rate of dissolution while the areas having undergone an adequate dose of exposure quickly become alkali soluble. Additionally, very high etching resistance is exerted by virtue of the robust aliphatic ring positioned on the backbone of the polymer and a plurality of aliphatic rings contained in the protective group of formula (1b).